In recent years, audio players that use semiconductor memories as recording media have been developed. In many cases, encoders perform compression coding on music contents stored in advance in recording media, such as a CD, and the music contents are stored in semiconductor memories (CD ripping). Then, decoders reproduce the compressed audio contents stored in the semiconductor memories so that users of the audio players can enjoy music.
In these cases, the encoders need to have the following characteristics in view of the user's convenience, such as: (1) a lower bit rate in compression coding; and (2) faster processing speed in compression coding. The lower bit rate in compression coding allows even an inexpensive recording medium having a smaller recording capacity to record data for a long period of time. Furthermore, the faster processing speed in compression coding makes it possible to code many tunes within a shorter period of time.
In recent years, the MPEG AAC-SBR method (ISO/IEC14496-3) has been standardized as a coding method characterized by using a lower bit rate. FIG. 1 illustrates the basic principle of compression coding according to the AAC-SBR method. According to the AAC-SBR method, compression coding is performed on a signal in a lower frequency band, using the AAC method in a main layer (left side in FIG. 1), and compression coding is performed on a signal in a higher frequency band, using the SBR method in a sub-layer (right side in FIG. 1). Here, the SBR method is a method of replicating, modifying, and shaping a signal in a lower band. Thus, the SBR part in FIG. 1 can be coded at a very low bit rate.
FIG. 2 illustrates the basic structure of a bit stream according to the AAC-SBR method. An AAC part (AAC data) and an SBR part (SBR data) are stored in each of audio frames divided at predetermined time intervals, and header information (AAC header and SBR header) is placed at the head of each of the audio frames. Here, as long as an SBR header that is a header of the SBR part is inserted once every several frames, the SBR header does not necessarily have to be present in each audio frame in view of reduction in a bit rate. The MPEG standard suggests insertion of an SBR header 2 times per second. The reason of such regular insertion is as follows. When the aforementioned methods are used in broadcasting, for example, input in a decoder does not always start from the head of a bit stream. Under such a condition, when an SBR header is inserted only in a head of a bit stream but SBR headers are not inserted at regular intervals, the SBR part cannot be reproduced to the end of the bit stream. Similarly, reproduction of a bit stream stored in advance does not start from the head of the bit stream. For example, in the case of reproduction from a 40-second time point, the SBR part cannot be reproduced without regular insertion of the SBR headers.
In order to accelerate the compression coding, a great many techniques that cause encoders to perform faster processing have been developed. Since these techniques are out of the scope of the present invention, the description is omitted here. On the other hand, several processing methods have been devised in view of the acceleration of the compression coding in an encode system. FIG. 3 illustrates a system configuration for processing music signals in a CD at faster speed.
First, a music signal stored in a CD is transmitted to an input pulse code modulation (PCM) buffer. Next, a signal processor, that is a digital signal processor (DSP) obtains the music signal from the input PCM buffer, and performs compression coding on the music signal. The resulting bit stream obtained by the compression coding is stored through an output bit stream buffer in an external memory, such as a SD memory card. The series of operations are controlled by a microcomputer controller. For example, when music signals that are stored in the CD and that correspond to 10 tunes are coded, the music signals are sequentially inputted into the input PCM buffer without any interval between the tunes. Accordingly, the signal processor sequentially codes the music signals without detecting an interval between the tunes. Such processing is performed when the signal processor sequentially codes one long tune corresponding to 10 tunes in a length of time. The resulting output bit stream coded and stored in the output bit stream buffer is divided, according to the tune length information originally stored in the CD (minutes and seconds), into tunes, such as into the first track and the second track (See Patent Reference 1). Such processing is performed because coding several tunes at one time enables the signal processor to perform processing faster than coding tunes one-by-one.
Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2004-101638